1. Field of the Invention
The present invention relates to elastic wave devices that utilize elastic waves, such as surface acoustic waves or boundary acoustic waves. More specifically, the present invention relates to elastic wave devices including a plurality of longitudinally-coupled-resonator-type elastic wave filters that are connected with each other.
2. Description of the Related Art
An RF-stage duplexer of a mobile telephone device includes a transmission-side filter and a reception-side filter. A duplexer in which a transmission-side filter and a reception-side filter are defined by elastic wave filters is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2008-118277.
FIG. 12 is a plan view schematically illustrating the electrode structure of the duplexer described in Japanese Unexamined Patent Application Publication No. 2008-118277. A duplexer 1001 includes the illustrated electrode structure provided on a piezoelectric substrate 1002.
Specifically, a reception-side filter 1006 is connected between an antenna terminal 1003 and first and second balanced terminals 1004 and 1005, which function as first and second reception terminals. Furthermore, a transmission-side filter 1008 is connected between the antenna terminal 1003 and a transmission terminal 1007. The transmission-side filter 1008 is configured as a ladder circuit including a plurality of elastic wave resonators.
In contrast, the reception-side filter 1006 includes an unbalanced terminal 1009, which is connected to the antenna terminal 1003, and the first and second balanced terminals 1004 and 1005. Here, a first elastic wave filter 1011 is connected to the unbalanced terminal 1009 via a single-port elastic wave resonator 1010. The first elastic wave filter 1011 includes first to third IDT electrodes 1013 to 1015 and is a three-IDT longitudinally-coupled-resonator-type surface acoustic wave filter. Similarly, a second elastic wave filter 1012 is also a three-IDT longitudinally-coupled-resonator-type surface elastic wave filter including first to third IDT electrodes 1016 to 1018.
One end of the second IDT electrode 1014 is connected to the unbalanced terminal 1009 via the elastic wave resonator 1010 and the other end thereof is connected to a ground terminal. One end of the first IDT electrode 1013 and one end of the third IDT electrode 1015 are connected to each other and the other ends thereof are respectively connected to one end of the first IDT electrode 1016 and one end of the third IDT electrode 1018 of the second elastic wave filter 1012. The other ends of the first and third IDT electrodes 1016 and 1018 are connected to each other. One end of the second IDT electrode 1017 is connected to the first balanced terminal 1004 and the other end thereof is connected to the second balanced terminal 1005. Thus, a balanced-unbalanced conversion function is provided.
This type of filter having a balanced-unbalanced conversion function is a float-balance-type elastic wave filter.
On the other hand, as illustrated in FIG. 13, a neutral-point-balance-type elastic wave filter, which is not a float-balance-type elastic wave filter, is also known. In the elastic wave filter 1101 illustrated in FIG. 12, the first and second elastic wave filters 1111 and 1112 are connected in parallel with the unbalanced terminal 1009. The third and fourth elastic wave filters 1113 and 1114 are cascade connected to the first and second elastic wave filters 1111 and 1112, respectively, and the third and fourth elastic wave filters 1113 and 1114 are respectively connected to the first and second balanced terminals 1004 and 1005.
A float-balance-type elastic wave filter, such as the reception-side filter 1006 illustrated in FIG. 12, is capable of reducing loss to a greater extent than a neutral-point balance-type filter. However, in a float-balance-type elastic wave filter, there is a problem in that the degree of balance between signals at the first and second balanced terminals 1004 and 1005 is worse than that in a neutral-point-balance-type filter. This is due to the fact that the first balanced terminal 1004 and the second balanced terminal 1005 are affected differently by floating capacitances and other factors, because in a float-balance-type filter, wiring connected to the first and second balanced terminals 1004 and 1005 cannot be made symmetrical, in contrast to in a neutral-point-balance filter. Consequently, in the duplexer 1001 illustrated in FIG. 12, there is a problem in that the reception-side filter 1006 is not sufficiently isolated from the pass band of the transmission-side filter 1008.